Frequency-scan traveling wave antenna

ABSTRACT

A frequency-scan traveling wave antenna receives radio frequency (RF) energy at an input port, passes the energy through a quarter wave transformer to a first radiator element in an array of radiator elements, each pair of radiator elements being connected by a respective delay line. Each radiator element includes an input port of known characteristic impedance connected to an impedance matching section which compensates for that element&#39;s radiated power. Each radiator element has an output port with a section of transmission line disposed between a main radiator section of said radiator element and said output port, and impedance matched to the output port. In one embodiment, each delay line includes a plurality of delay line sections, with adjacent delay line sections being mutually perpendicular. In a second embodiment, each delay line includes a plurality of delay line sections disposed in meandering form.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a frequency-scan travelingwave antenna, and more particularly to a frequency-scan traveling waveantenna which operates at frequencies in the millimeter wave range.

2. Description of the Prior Art

Frequency-scan traveling wave antennas are known in the art. Typically,uses of such antennas include their use in collision avoidance radar andin imaging radar. However, such frequency-scan traveling wave antennasare burdened by disadvantages, and those disadvantages adversely affectthe quality of their performance. This interferes with the use of suchantennas as an effective component of collision avoidance radar systemsand imaging radar systems.

Problems with traveling wave antennas of the prior art include thefollowing: inferior radiating element design, inadequate impedancematch, insufficient power weighting accuracy, and inferior phasecoherence across an array.

Accordingly, there is a need for the development of a frequency-scantraveling wave antenna which enables frequency scanning of an antennabeam at millimeter wave frequencies, where the beam and the side lobelevels are at an improved degree of controllability and performance overthose of the antennas of the prior art. Moreover, there is a need forthe development of a frequency-scan traveling wave antenna having anoptimized radiating element design, improved impedance match, improvedpower weighting accuracy, and better phase coherence across the array,thereby enabling accurate design of beam width and side lobe level.

SUMMARY OF THE INVENTION

The present invention generally relates to a frequency-scan travelingwave antenna, and more particularly, to a frequency-scan traveling waveantenna which operates at millimeter wave frequencies. In particular,the present invention provides a high performance traveling wave antennawhich scans by frequency, and which can be used effectively as acomponent of collision avoidance radar systems and imaging radarsystems.

The frequency-scan traveling wave antenna of the present invention usesan impedance matching transformer at the input of each radiator elementto compensate for radiated power in the element, and allows the inputimpedance of the radiator element to be the same as its outputimpedance. This is in contrast to conventional traveling wave radiators,which do not use impedance matching to correct for impedance shiftsarising from radiated energy. The specific use of impedance matching ateach element in the traveling wave frequency scan antenna of the presentinvention reduces reflections, and provides better control over arraydesign.

The present invention also provides an arrangement of radiators anddelay lines which are geometrically symmetric, thereby balancing strayradiation and minimizing the amount of degradation of the beam patternand the side lobe levels due to radiation from the delay lines betweenthe radiator elements.

To summarize, the traveling wave antenna of the present invention usesimpedance matched radiator elements and a symmetric layout. This enablesthe invention to achieve an accurate degree of control over powerweighting at each radiator element, resulting in improved side lobelevels and a narrow beam at millimeter wave frequencies.

Therefore, it is a primary object of the present invention to provide afrequency-scan traveling wave antenna operating at millimeter wavefrequencies.

It is an additional object of the present invention to provide a highperformance antenna which scans by frequency, and which can be usedeffectively in collision avoidance radar systems and imaging radarsystems.

It is an additional object of the present invention to provide afrequency-scan traveling wave antenna having an optimized radiatingelement design.

It is an additional object of the present invention to provide afrequency-scan traveling wave antenna having improved impedance match.

It is an additional object of the present invention to provide afrequency-scan traveling wave antenna having improved power weightingaccuracy.

It is an additional object of the present invention to provide afrequency-scan traveling wave antenna providing better phase coherenceacross the array, thereby enabling accurate design of beam width andside lobe level.

It is an additional object of the present invention to provide afrequency-scan traveling wave antenna which employs an arrangement ofradiators and delay lines which is geometrically symmetric, therebybalancing stray radiation and minimizing the amount of degradation ofthe beam pattern and the side lobe levels due to radiation from thedelay lines between the radiators.

The above and other objects, and the nature of the invention, will bemore clearly understood by reference to the following detaileddescription, the associated drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a preferred traveling waveantenna array employed in the present invention.

FIG. 2A is a detailed diagram of a single series feed radiating elementutilized to construct the preferred traveling wave antenna array of FIG.1.

FIG. 2B is a detailed diagram of the delay line between radiatorelements in the frequency-scan traveling wave antenna of the presentinvention.

FIG. 3 is a graphical illustration of the relative power weights acrossthe radiator elements of the array.

FIG. 4 is a graphical illustration of the calculated far-field patternof an antenna (normalized magnitude in dB) verses azimuth angle (indegrees).

FIG. 5 is a diagrammatic representation of an alternative embodiment ofthe present invention, in which the delay line between neighboringelements is meandering.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in more detail with reference to thevarious figures of the drawings.

FIG. 1 is a diagrammatic representation of a preferred traveling waveantenna array employed in the present invention. As seen therein, thefrequency-scan traveling wave antenna 40 of the present inventioncomprises the following elements: input port 1; quarter wave transformer2; first radiator element 3; first delay line 4; second radiator element5; second delay line 6; third radiator element 7; third delay line 8;fourth radiator element 9; intervening delay lines and radiator elementsgenerally indicated by reference numeral 10; penultimate radiatorelement 11; final delay line 12; final radiator element 13; and outputport 14.

The composition of each radiator element 3, 5, 7, 9, 11 and 13 (and anyintervening radiator elements) will be described in detail below withreference to FIG. 2A, while the composition of each delay line 4, 6, 8,and 12 (and any intervening delay lines) will be described in detailbelow with reference to FIG. 2B. Output port 14 comprises an impedancematched load which may be either a matched resistor or an impedancematched port.

As with all passive antennas, the traveling wave antenna shown in FIG. 1is reciprocal; it can be used as either a transmission or a receptiondevice. For simplicity, the following detailed description relates toits operation as a transmitter. It will be recognized by those ofordinary skill in the art that, if the antenna described herein is usedas a reception antenna, each element listed and described serves thesame purpose in the reception mode as in the transmit mode, but only thedirection of signal flow is reversed.

Referring to FIG. 1, in operation, radio frequency (RF) energy entersthe traveling wave antenna 40 at input port 1 from an external impedance(of, for example, 50 ohms.) The RF energy is provided, via quarter wavetransformer 2, to the first radiator element 3 having an electricallength Le. Radiator element 3 is connected to delay line 4 havingelectrical length Ld. The series of radiator elements 5,7,9,11 and 13and delay lines 4,6,8, 10 and 12 (with intervening radiators and delaylines generally indicated by reference numeral 10 in FIG. 1) is reachedby the RF energy. Final radiator 13 is connected to an impedance matchedload or output port 14, which may be either a matched resistor or animpedance-matched port.

FIG. 2A is a detailed diagram of a single series feed radiating elementutilized to implement the radiating elements 3, 5, 7, 9, 11 and 13 (andintervening radiating elements) of FIG. 1. As seen therein, a singleseries feed radiating element comprises: an input port 20 of knowncharacteristic impedance; an impedance matching section 21; a mainradiator section 22; a transmission line section 23; and an output port24.

Further referring to FIG. 2A, the transmission line section 23 isimpedance matched to output port 24, and is included to set theelectrical length of the overall radiator element to one full wavelengthLe. The impedance matching section 21 adjusts the input impedance of theoverall radiator so that it is the same as the impedance of the outputport 24.

FIG. 2B is a detailed diagram of the delay line used to implement delaylines 4, 6, 8 and 12 (and intervening delay lines) in the frequency-scantraveling wave antenna of FIG. 1. As seen therein, the delay lineconsists of a section of transmission line having an input port 25 andan output port 31 connected by a conventional transmission line ofuniform impedance composed of transmission line sections 26, 28 and 30.A mitered corner 27 is provided between sections 26 and 28, and amitered corner 29 is provided between sections 28 and 30. Miteredcorners 27 and 29 are of conventional optimal shape so as to maintainuniform impedance along the delay line. Input port 25 and output port 31are separated by a spacing D, which consequently sets the spacingbetween radiator elements in the frequency scan array. All delay linesin the array are of the same electrical length Ld between their inputport 25 and their output port 31 so as to maintain a consistent phase oneach radiator element throughout the array. The preferred electricallength is one of the series (3/2, 5/2, 7/2, . . . ) wavelengths at thecenter frequency of operation. The radiating beam points in aperpendicular direction relative to the plane of the array at the centerfrequency of operation.

FIG. 3 is a graphical illustration of the relative power weights acrossthe radiator elements of the array. In particular, FIG. 3 shows therelative power weights across the radiator elements: as determined usingconventional means of calculation, known as N-Taylor taper weighting,thereby designing the beam width of the antenna. The present inventionprovides means for accurately yielding the sought power weights in thephysical design of the traveling wave antenna.

FIG. 4 is a graphical illustration of the calculated far-field patternof an antenna (normalized magnitude in dB) verses azimuth angle (indegrees). That is, FIG. 4 is a graphical illustration of the calculatedantenna pattern at the center design frequency when the antenna isoriented in the horizontal direction.

A further description of the operation of the invention, referring tothe preceding figures, is now provided. Referring to FIG. 1, RF energyenters input port 1, and propagates through each radiator element 3, 5,7, 9, 11 and 13 (and intervening radiator elements) of the array. Aportion of the RF energy entering a particular radiator element exitsfrom the exposed surface of that radiator element as RF radiation. Anyresidual energy not radiated in the radiator elements 3, 5, 7, 9, 11 and13 (and intervening radiator elements) reaches the end of the array, andis dissipated in the output port 14 so as not to reflect backward alongthe array. The phase of the radiation from each radiator element 3, 5,7, 9, 11 and 13 (and intervening radiator elements) depends on the phaseof the current passing through each element. Since the flow of currentreverses from top to bottom and then from bottom to top betweenneighboring radiator elements in the array, the electrical phase betweenelements at a center design frequency must be equal to (N+1/2)wavelengths for N=1,2,3,etc., so as to maintain all radiator elements inphase at the design frequency. When all radiator elements are in phase,the beam formed by the array will point perpendicular to its face. Thepolarization of the resulting beam is perpendicular to the length of theone-dimensional array.

When the operation frequency is slightly above the center designfrequency of the array, the electrical phase between neighboringradiator elements will be slightly greater than (N+1/2) wavelengths. Theextra phase will cause further elements along the array to have aleading phase in their radiation. The net result is a beam which nolonger radiates perpendicularly off the face of the antenna, but whichpoints off the antenna toward the input end of the array. The angulardegree of pointing away from the perpendicular direction increases indirect proportion to the difference between the frequency and the centerdesign frequency; that is, the further the frequency is above the centerdesign frequency, the greater the angular degree by which the beampoints away from the perpendicular direction.

When the operation frequency is slightly below the center designfrequency of the array, the electrical phase between neighboringradiator elements will be slightly less than (N+1/2) wavelengths. Thereduced phase will cause further elements along the array to have alagging phase in their radiation. The net result is a beam which nolonger radiates in the perpendicular direction off the face of theantenna, but rather points off the antenna away from the input end ofthe array. The angular degree of pointing away from the perpendicularincreases in direct proportion to the amount by which the frequency isbelow the center design frequency; that is, the further the frequency isbelow the center design frequency, the greater the angular degree bywhich the beam points away from the perpendicular.

When the value of N is a large integer, the amount of angular scanningwith frequency is increased. By having the delay line elements 4, 6, 8and 12 (and intervening delay lines) symmetrically placed about theradiator elements 3, 5, 7, 9, 11 and 13 (and intervening radiatorelements), stray RF energy radiating from the delay line sectionsbalance each other, thereby having a reduced effect on the radiatorelements 3, 5, 7, 9, 11 and 13 (and intervening radiator elements) ofthe frequency scan antenna array 40.

An array as described above is constructed in six steps, the first threeof which are as follows: (1) generate a termination patch having aninput impedance Zo and at the design frequency using full wave numericalsimulation; (2) generate a quarter-wave input matching transformer ofinput impedance Zo/2 and output impedance Zo at the design frequencyusing full wave simulation; and (3) build a small database of impedancematched and correctly phased radiator elements at the design frequencyusing full wave simulation (in this regard, the structural templateshown in FIG. 2A is applicable).

The entries in the small database are as follows:

W(radiator) L (radiator) L(match) W(match) L(phase) P(radiated) whereW(radiator) is the width of main radiator section 22 (FIG. 2A)perpendicular to the direction of RF signal propagation; L(radiator) isthe length of main radiation section 22 along the direction of RF signalpropagation; W(match) is the width of impedance matching section 21;L(match) is the length of impedance matching section 21; L(phase) is thelength of transmission line section 23; and P(radiated) is the powerradiated, as determined by full wave simulation. The database is builtby optimizing radiator elements of different W(radiator) values forminimum S11 reflection S-parameter.

The method of designing the array continues with the following threesteps: (4) generating a set of power weights for each element usingconventional theory (for example, FIG. 3); (5) finding entries in thedatabase both above and below the sought power weight; and (6) from theselected entries, interpolating a specific radiator design for use inthe overall array design and fabrication.

FIG. 5 is a diagrammatic representation of an alternative embodiment ofthe present invention, in which the delay line between neighboringradiator elements is meandering. As seen therein, the alternateembodiment of the frequency-scan traveling wave antenna compriseselements generally corresponding to the elements shown in the firstembodiment of FIG. 1. Thus, the elements are as follows: input port 51;quarter wave transformer 52; radiator element 53; meandering delay line54; radiator element 55; meandering delay line 56; radiator element 57;meandering delay line 58; radiator element 59; various intervening delaylines and radiator elements eliminated for simplicity, but generallyindicated by reference numeral 60; penultimate radiator element 61;final delay line 62; final radiator element 63; and output port 64.Operation of the embodiment of FIG. 5 is generally the same as theoperation of the embodiment of FIG. 1 described above, and thus furtherdetail will not be provided.

Various modifications to the invention disclosed therein can beimplemented. For example, the electrical phase between the center ofneighboring radiator elements is, preferably, one of the series (N+1/2)wavelengths. Thus the delay lines can be altered in length toaccommodate this factor, while still preserving both array symmetry andcoherent formation of a radiating beam.

A one-dimensional frequency array as shown may be used as a component ina two-dimensional array to achieve two-dimensional beam shaping andscanning control. It should be noted that the scanning will still be inone axis.

The delay line between neighboring elements may be meandered, as shownin the embodiment of FIG. 5, in order to achieve better packing density.This is especially advantageous when the integer value of the parameterN is large.

The number of elements in the one-dimensional array can be selected asrequired to achieve a required degree of beam width narrowing.

The spacing between the center of radiator elements, indicated in FIG. 1above by the parameter “D”, is chosen for close packing of radiatorelements. However, it may be as large as 1.5 wavelengths at the centerdesign frequency. Although it is preferred that neighboring radiators bespaced at a uniform spacing of D, small deviations from this uniformitycriterion (up to 20% variation) is possible without serious degradationof antenna performance.

Finally, the design of the present invention has been developed andtested at 39 GHz. Nevertheless, the design process is such that thepresent invention can be implemented by arrangements representing ascaling up to 100 GHz. and a scaling down to 10 GHz, or even lower, ifthe system can support the required larger dimensions.

As described above, the present invention employs an impedance matchingsection 21 (FIG. 2A) at the input of each main radiator section 22 tocompensate for radiated power in the radiator elements 3, 5, 7, 9, 11and 13 (and any intervening radiator elements), and to allow the inputimpedance of each radiator element to be the same as its outputimpedance. Conventional traveling wave radiators have not used impedancematching to correct for impedance shifts arising from radiated energy.The specific use of impedance matching at the input of each radiatorelement in the traveling wave frequency scan antenna reduces reflectionsand provides better control over the array design, and is a unique andnovel feature of the present invention.

In the invention, the arrangement of radiator elements and delay linesis geometrically symmetric, thereby balancing stray radiation, andminimizing the amount of degradation of beam pattern and side lobelevels due to radiation from the delay lines between the radiatorelements.

Finally, the traveling wave antenna of the present invention usesimpedance matched radiator elements and a symmetric layout, and thisenables one to obtain an accurate degree of control over power weightingat each radiator element, resulting in improved side lobe levels andnarrow beam at millimeter wave frequencies.

While preferred forms and arrangements have been shown in illustratingthe invention, it is to be understood that various changes andmodifications may be made without departing from the spirit and scope ofthis disclosure.

What is claimed is:
 1. A frequency-scan traveling wave antenna,comprising a plurality of paired radiator elements, each pair ofradiator elements being connected by a respective delay line; each ofsaid radiator elements comprising an input port, a main radiator, andimpedance matching means connected between said input port and said mainradiator for compensating for radiated power in said radiator elements,and for matching an input impedance of said radiator elements to anoutput impedance of said radiator elements.
 2. The frequency-scantraveling wave antenna of claim 1, further comprising a quarter-wavetransformer connected to said input port of a first one of said radiatorelements.
 3. The frequency-scan traveling wave antenna of claim 1,wherein each of said radiator elements includes an output port and asection of transmission line connected between said main radiator andsaid output port, said section of transmission line being impedancematched to said output port.
 4. The frequency-scan traveling waveantenna of claim 3, wherein said section of transmission line sets anelectrical length of said radiator elements to one full wavelength. 5.The frequency-scan traveling wave antenna of claim 1, wherein saidplurality of radiator elements and said delay line disposed betweenrespective pairs of said radiator elements form a geometricallysymmetric arrangement, thereby balancing stray radiation and minimizingan amount of degradation of a beam pattern and side lobe levels due toradiation from said delay line between said radiator elements.
 6. Thefrequency-scan traveling wave antenna of claim 1, wherein each of saiddelay line comprises an input port, an output port, and a plurality oftransmission line sections extending between said input port and saidoutput port, said plurality of transmission line sections having uniformimpedance.
 7. The frequency-scan traveling wave antenna of claim 6,wherein each of said transmission line of each of said delay line isconnected to an adjacent transmission line section by a mitered cornerso as to maintain uniform impedance along each of said delay line. 8.The frequency-scan traveling wave antenna of claim 1, wherein each ofsaid delay line has the same electrical length.
 9. The frequency-scantraveling wave antenna of claim 8, wherein each of said delay line hasan electrical length equal to one of the series (3/2, 5/2, 7/2, ...)wavelengths at a center frequency of operation.
 10. The frequency-scantraveling wave antenna of claim 1, wherein an electrical phase betweenthe center of each respective pair of radiator elements is one of theseries (N+1/2) wavelengths for positive integer N.
 11. Thefrequency-scan traveling wave antenna of claim 1, wherein each of saiddelay line is meandered to achieve better packing density.
 12. Thefrequency-scan traveling wave antenna of claim 1, wherein a spacingbetween the center of adjacent radiators is no greater than 1.5wavelengths at a center design frequency.